This invention relates to the dyeing of textile fibers and fabrics. More specifically, described is a process for rapid dyeing of textiles from entrained high-boiling solvents which is conducted on a continuous basis and optionally simultaneously heat-sets the fabric while the fabric is being dyed. Unlike prior procedures, no special precautions are required to isolate the dye composition from the atmosphere. This process results in level, uniform dyeing free from end-to-end variations and side-to-center-to-side shade changes. Deterioration of the dyes, easily detected by color change, was not observed. Moreover, shade matches are readily obtained.
Dyeing processes currently in use in the textile industry are often limited in performance by several fundamental factors. Aqueous dyeing processes at atmospheric pressure are limited in that the dyeing temperature is necessarily maintained below the boiling point of water. Since dye penetration of the fiber is often slow at such temperatures, batch processes with long dwell times are common. Pressurized aqueous dyeing processes offer the advantage of increased dyeing temperature and thus, dyeing rate. However, the design of pressurized vessels restricts such processes to operation in the batch mode. Additionally, dyeing temperatures are still relatively low so that long dwell times are common. In order to minimize the number and size of costly batch dyeing machines required, such machines are designed to maximize the amount of fabric that can be processed. Thus, batch processes often operate so that the fabric is dyed in rope form. Rope dyeing promotes non-uniform coloring of the fabric due to inconsistent contact with dye liquor across the fabric width. Batch processes often require large volumes of dye liquor, which imposes additional costs for heating the dye liquor and for disposal of waste liquors.
A continuous, open-width dyeing process which overcomes the temperature limitation of aqueous processes is the well-known thermosol process. One disadvantage of this process is that it is suited for only a small selection of dyestuffs and fibers. Additionally, the thermosol process is energy intensive in that the fabric must be dried before dyeing can occur. Another disadvantage of the drying step is that poor uniformity often occurs due to dye migration resulting from uneven drying.
Continuous dyeing processes in general suffer from end-to-end shade variations due to depletion of dye liquors, commonly called tailing, and from difficulty in adjusting and controlling dye liquor concentrations. A related problem is one of predictability--dyeing recipes formulated in the laboratory often produce different results in the production process.
A problem common to textile dyeing processes in general is that the fabric heat history prior to dyeing affects the shade obtained upon dyeing.
Another type of dyeing procedure uses a non-aqueous system that operates at elevated temperatures. The non-aqueous dye compositions consist of a dye or several dyes dissolved or usually dispersed in a high-boiling, nonionic, organic solvent. This class of solvents is described in various U.S. patents, of which U.S. Pat. No. 4,293,305 to Wilson is typical. Additional related patents are identified in more detail below.
More recently, Clifford in U.S. Pat. No. 4,550,529 proposes to dye shaped articles using these non-aqueous dyeing compositions composed of a dyestuff and a high-boiling, nonionic solvent in a non-reactive environment in order to overcome problems of stability of the dye composition at elevated temperatures. Specifically, Clifford states that the dye composition based upon certain high-boiling aromatic esters or cycloaliphatic diesters does not remain stable over a period of time when used at high temperatures in an air atmosphere and, in fact, significant degradation of the dye composition can be expected after only a few hours of use. These dye compositions are frequently used and recirculated while being maintained at temperatures in the range of from 350.degree.-380.degree. F.
In related published European Application No. 159,876 to Pensa et al., the importance of a non-reactive environment, such as achieved with Freon.RTM. TF (DuPont), is demonstrated in a series of comparative examples. This patent reports significant loss of color strength for high-boiling aromatic ester dye compositions after being exposed to the air for a period of four hours.
Thus, the art clearly teaches the importance of using such high-boiling dye compositions in a non-reactive, inert environment.
We have discovered a process by which textile articles are rapidly and continuously dyed in open width, and optionally simultaneously heat set, using dye compositions based upon high-boiling, nonionic organic solvents in a normal air atmosphere. Special precautions for maintaining a non-reactive environment are not required.
In the process of the present invention, the dye composition is preferably prepared, held, and applied to the textile at a relatively low temperature, typically 200 to 250.degree. F., at which degradation of the dye composition is negligible. A minimal amount of dye composition, typically less than one pound of dye composition per pound of textile, is applied to the textile, preferably by padding, and is indirectly heated with the textile to the dyeing temperature, e.g., by impinging hot air. In this manner, a minimum amount of dye composition is exposed to dyeing temperature and for only a very short period of time, typically 30 to 60 seconds.
By contrast, in the preferred embodiment of the process of EPA 159,876, the dye composition is maintained at the dyeing temperature (page 28, line 19) during the entire operation of the process. Thus, the dye composition is continuously maintained at a temperature at which it is susceptible to degradation. Also, a thin continuous film of dye composition that flows over all of the surfaces of the textile to be dyed illustrates that an excess of dye composition is contacted with the material and is necessarily exposed to the dyeing temperature.
Rapid dyeing from high-boiling solvents has previously been disclosed by Hermes of Martin Processing. U.S. Pat. No. 4,047,889 discloses a general process including scouring, recovery, and recycling. The entire process is conducted under closed conditions.
U.S. Pat. No. 4,055,971, also to Hermes, describes specific apparatus for implementing the process disclosed in U.S. Pat. No. 4,047,889 above. Also, U.S. Pat. No. 4,047,889 refers to dyeing apparatus disclosed in U.S. Pat. No. 3,558,260. The disclosures refer to two methods of contacting textiles with dye compositions: immersion, in which dye is transferred from dyebath to an immersed textile; and cascade, in which dye is transferred from dye composition continuously sprayed over the textile. These two contacting schemes are dynamically similar in that the textile is contacted with a large excess of dye liquor which changes only very slowly in concentration with time. By contrast, the entrainment method of contact of the present invention utilizes a carefully controlled minimum amount of dye composition which changes substantially in concentration as dye is transferred from the entrained dyebath to the textile. Due to the concentration change, the entrainment dyeing process is not easily understood without consideration of fundamental mass transfer principles. To our knowledge, the feasibility and advantages of contact by entrainment in solvent dyeing have not previously been recognized.